When you make a reservation, your school should send you a detailed list of things to bring. When packing, remember that during the summer months of July and August it can get quite hot, with temperatures reaching 25°C and more. You will need clothes that you could play sport in, but you shouldn't need to bring a dictionary or phrase book, as most schools will provide these for you.
Further details of Our World English Schools' courses are available from:
www.ourworldenglish.com